The present invention relates to integrated circuits, and in particular to a three-level semiconductor balun and method for creating the same.
The use of twisted pairs of copper wires to form coupled transmission line elements is well known. These transmission line elements may be used to create baluns, balanced and unbalanced transformers and current and voltage inverters. Examples of the use of conventional transmission line elements are presented in C. L. Ruthroff, xe2x80x9cSome Broad-Band Transformers,xe2x80x9d Proceedings of the IRE (Institute for Radio Engineers), vol. 47, pp. 1337-1342 (August 1959), which is incorporated herein by reference. These transmission line elements are typically found in forms that are useful in frequency bands through UHF.
The use of such transmission line elements in integrated circuits such as RF power amplifiers and low noise amplifiers is desirable. However, the incorporation of off-chip devices such as these conventional transmission line elements into RF devices such as cellular telephones is not competitive due to size and cost. Moreover, conventional coupled transmission line elements are not suitable for use in the desired frequency range.
Therefore, a need has arisen for a coupled transmission line element that addresses the disadvantages and deficiencies of the prior art. In particular, a need has arisen for a low-loss balun suitable for integration in RF integrated circuits.
Accordingly, a three-level semiconductor balun is disclosed. In one embodiment, the balun includes a first spiral-shaped transmission line overlying a substrate. The first transmission line has first and second ends. A second spiral-shaped transmission line is substantially vertically aligned with the first transmission line. The second transmission line has a first end electrically connected to the second end of the first transmission line. A third spiral-shaped transmission line is substantially vertically aligned with the first and second transmission lines. The third transmission line has a first end electrically connected to a second end of the second transmission line. In one embodiment, a first balanced-side terminal is electrically connected to the first end of the first transmission line, a second balanced-side is terminal electrically connected to the first end of the third transmission line, and an unbalanced-side terminal is electrically connected to the second end of the third transmission line.
In another aspect of the present invention, a method for creating a balun on a semiconductor substrate is disclosed. The method includes forming a first transmission line on the substrate, forming a second transmission line substantially overlying the first transmission line, the second transmission line having a first end electrically connected to the second end of the first transmission line, and forming a third transmission line substantially overlying the first and second transmission lines, the third transmission line having a first end electrically connected to a second end of the second transmission line.
An advantage of the present invention is that the balun may be integrated on the same chip with other RF circuit components. Another advantage of the present invention is that the balun is suitable for use at higher frequencies than most conventional (non-integrated) baluns.